The field of the present invention is time-sharing FM radar systems and, in particular, time-sharing FM radar systems for use in collision avoidance systems.
A radar system, which is mounted on a vehicle such as an automobile and used in conjunction with an alarm system to detect and warn of potential frontal and rearend collisions, can be implemented in various ways. For example, such a radar system can be implemented using either a pulse radar system, which transmits and receives pulsive electromagnetic waves, or an FM radar system which transmits and receives an FM signal. However, it has been recognized that FM radar systems are preferred over pulse radar system, because radar systems used in collision avoidance systems must have a minimum detection range of several decimeters. In FM radar systems, it is preferred to generate a frequency modulated (FM) signal having a frequency varying linearly with time. The generated FM signal is divided into two parts. One part is radiated from an antenna, and the other part is supplied to one input terminal of a mixer as a local FM signal. The radiated beam, if reflected by an object (hereinafter called a "target"), will produce a return beam. The return beam may then be received by an antenna and supplied to a second input terminal of the mixer to be mixed with the local FM signal to produce a beat signal. By detecting the frequency of the beat signal, the phase shift or timing delay between the radiated beam and the return beam may be determined. As the timing delay represents the propagation time required for the radiated beam to travel to and return from the target, the timing delay may be used to calculate the range or distance to the target.
In the prior art FM radar systems described above, it is preferred to use beams in a frequency range above 30 GHz and having a wave length on the order of a millimeter (mm), which beams are rapidly attenuated upon propagation, to avoid interference between microwave transmission systems already in existence. This is because the longest range to be detected is about several hundred meters at most.
FM radar systems may also be used to determine a direction to a target. Such systems generally utilize a plurality of trnsceivers to detect both a direction to a target and a range to the target. Each antenna is arranged to radiate a beam having substantially the same pattern (or directivity) as the beams radiated by the other antennas. However, each antenna radiates a beam in a slightly different direction from the beam radiated by the other antennas. FM signals of substantially equal amplitude are supplied to the antennas and are radiated therefrom. Those beams, which are reflected by a target, produce return beams, and the return beams may be received by either the same or other antennas. The direction to the target is calculated based upon a ratio or ratios of the amplitudes of the return beams.
There are two ways to avoid interference between the transceivers. One way is to allocate FM signals of different frequencies to each of the transceivers or channels, and the other way is to distribute FM signals of the same frequency to each of the transceivers in different timing. The latter technique, the so-called time-sharing technique, is preferred over the former technique, the so-called frequency-sharing technique, because the latter technique requires a reduced frequency range for operation.
As for the issue of how to implement each of the transceivers, there are two ways. One way is to use a dedicated transmitting antenna and a dedicated receiving antenna separately. The other way is to use only one antenna which is commonly used for both transmitting (radiating) and receiving. In the latter case, FM signals to be transmitted and return beams received are separated using a circulator. The latter antenna system, the so-called transceiving common antenna system, is preferred over the former type, the so-called dedicated antenna system, because the numbers of antennas required can be reduced. This results in a reduction in the overall system size and manufacturing costs. This is especially important in an FM radar system, wherein a relatively large number of antennas are used to detect both a direction to a target and a range to the target. An exemplary time-sharing/common antenna type FM radar system using mm wave FM signals is disclosed in U.S. Pat. No. 5,181,037.
As is shown in FIG. 6, a prior art time-sharing FM radar system may comprise four transceiving common antennas 110a-110d, an FM signal generating unit 120, a transmitting unit 130, a receiving unit 140 and a detection and control unit 150. The FM signal generating unit 120 may comprise a 20 GHz band voltage controlled oscillator 121 including a Gunn diode and sweeper circuit 122. The frequency of the microwave band FM signal generated in the FM signal generating unit 120 is varied with time and comprises a saw tooth wave form as shown in the timing chart of FIG. 7. The FM signal is divided into two parts by power divider 123. One part is supplied to a transmitting switching circuit 131 in the transmitting unit 130, and another part is supplied to a local switching circuit 141 in the receiving unit 140.
The FM signal supplied to the transmitting unit 130 is distributed to each of four triple frequency multipliers 132a-132d successively through the transmitting switching circuit 131 which comprises a plurality of switching elements, such as PIN diodes, and produces transmitting waves TXa-TXd (mm waves of 60 Ghz). Transmitting waves TXa-TXd are shown in the timing chart of FIG. 7. Each of the transmitting waves TXa-TXd is supplied to one of the transceiving common antennas 110a-110d successively through one of the circulators 160a-160d, and each of the transmitting waves TXa-TXd is radiated by one of the transceiving common antennas 110a-110d.
Any of the FM signal beams, which are radiated from the transceiving common antennas 110a-110d and are reflected by a target, will produce return beams which, in turn, may be received by some or all of the transceiving common antennas 110a-110d. The received return beams are separated from the transmitting beams TXa-TXd by the circulators 160a-160d and supplied to the received signal input terminals of mixers 143a-143d. The FM signals supplied from power divider 123 to the transmitting unit are distributed to each of the triple frequency multipliers 142a-142d successively through a receiving switching circuit 141. The resulting signals are referred to herein as local signals Loa-Lod and are shown in the timing chart of FIG. 7. The receiving switching circuit 141 comprises a plurality of switching elements such as PIN diodes. Each of the local signals Loa-Lod is supplied to a local signal input terminal of each of the mixes 143a-143d successively. Beat signals BTa-BTd generated by the mixers 143a-143d are selected by a beat selector 144 and supplied to a detection circuit 150. The timing of operations within the various parts of the system, for example, within switching circuits 131 and 141, is controlled by the timing control signals output from the timing control circuit 152.
The prior art FM radar systems described above are somewhat inefficient, and it is recognized that those systems consume more power than is acceptable in a "battery powered" environment. More specifically, almost all of the electric power consumed by prior art FM radar systems is consumed by a single power amplifier which operates at the final stage of the FM signal generating circuit 121. Moreover, the power of the FM signal which is supplied to the PIN diodes comprising switching circuits 131 and 141 must be increased to compensate for a large on-state insertion loss of about 3 dB which arises when the PIN diodes are rendered conductive. This produces an increase in power consumption by the FM signal generating unit. However, the off-state insertion loss of the PIN diodes which results when the diodes are rendered nonconductive is not that large. Thus, the FM signal may leak into unexpected transmitting channels through the off-state PIN diodes and cause an increase of interference between channels.
Further, in prior art time-sharing FM radar systems, such as that shown in FIG. 6, an FM signal having a frequency of about 60 GHz is transmitted and received. This is done mainly to reduce the size of the antennas used in those systems. However, because it is difficult to operate switching circuits at such a high frequency after the signal frequency has been multiplied three times, the same number of frequency triple multipliers as the number of channels is used by the switching circuits in both the transmitting unit and the receiving unit. The utilization of multiple triple frequency multipliers adds unnecessary complexity to the FM radar systems of the prior art. Such utilization also adds unnecessarily to overall systems costs and maintenance requirements.
Further, in the FM radar system described above, a large number of antennas are required to increase both angular range of detection of the system as well as the accuracy in detecting a direction to a target. This also increases the size and manufacturing cost of the overall system. The reasons for this are as follows.
An exemplary FM radar system in which four beams, Ba, Bb, Bc, and Bd, are radiated from each of four transceiving common antennas A-D (not shown in the Figure) respectively is shown in FIG. 5. The antennas A-D have the same radiation and receiving pattern (directivity), and the antennas A-D are arranged to radiate beams Ba-Bd in slightly different directions such that the beams partially overlap. Further, assuming that a target has a size and location represented by circle 100 in FIG. 5, the amplitude level of the return beam radiated and received by antenna B (Lb) will be the highest value, and the amplitude level of the return beam radiated and received by antenna A (La) will be next highest value. In contrast, the amplitude levels of the return beams radiated and received by antennas C and D respectively (Lc and Ld) will both be zero. Those skilled in the art will appreciate that the direction to the target may be calculated based upon the amplitude levels La and Lb and the locations and directions of antennas A and B.
To increase the accuracy of the overall system, those skilled in the art will recognize that it is generally desirable to increase the number of return beams having a non-zero amplitude level. This increase in accuracy may be readily achieved by reducing the difference in direction between adjacent beams (i.e., by reducing the setting angles between the antennas). For example, the level of the return beam radiated and received by antenna C (Lc) may be converted to a non-zero value by reducing the setting angle between antennas B and C. Further, assuming that the directions of antennas A, B and C are .theta.a, .theta.b and .theta.c respectively, the direction to the target 8 can be calculated as follows. EQU .theta.=(La.multidot..theta.a+Lb.multidot..theta.b+Lc.multidot..theta.c) / (La+Lb+Lc)
In this way, a direction to a target can be detected more accurately. However, the reduction of setting angles between antennas leads to a reduction in the angular range of detection of the four beams Ba-Bd. As a result, an increased number of antennas is required if it is desired to increase both the accuracy of detection and the angular range of detection of the overall system. This results in an increase in overall system size and manufacturing costs.
Finally, in the time-sharing FM radar systems of the prior art (shown in FIG. 6), each of the mixers 143a-143d corresponds to one of the transceiving common antennas 110a-110d, and the outputs from the mixers are selected by beat selector 144. However, the levels of the beat signals and noises which are output from each of the mixers will be different as shown in FIG. 8. As an example, even if the same received return beams and local signals are supplied to each of the mixers, the output of the mixers will differ because each of the mixers 143a-143d will have mixing characteristics which differ from those of the other mixers. As a result, the beat signals generated by each mixer may differ substantially, and any direction calculated based upon those signals may be erroneous.